Optical glass fibers are usually coated with two superposed radiation-cured coatings, which together form a primary coating. The coating which contacts the glass surface is called the inner primary coating and the overlaying coating is called the outer primary coating.
The inner primary coating is usually a soft coating having a low glass transition temperature (hereinafter "Tg"), to provide resistance to microbending. Microbending can lead to attenuation of the signal transmission capability of the coated optical glass fiber and is therefore undesirable. The outer primary coating is typically a harder coating providing desired resistance to handling forces, such as those encountered when the coated fiber is cabled.
For the purpose of multi-channel transmission, optical glass fiber assemblies containing a plurality of coated optical fibers have been used. Examples of optical glass fiber assemblies include ribbon assemblies and cables. A typical optical glass fiber assembly is made of a plurality of coated optical glass fibers which are bonded together in a matrix material. For example, the matrix material can encase the optical glass fibers, or the matrix material can edge-bond the optical glass fibers together.
Optical glass fiber assemblies provide a modular design which simplifies the construction, installation and maintenance of optical glass fibers by eliminating the need to handle individual optical glass fibers.
Coated optical glass fibers for use in optical glass fiber assemblies are usually coated with an outer colored layer, called an ink coating, or alternatively a colorant is added to the outer primary coating to facilitate identification of the individual coated optical glass fibers. Thus, the Matrix material which binds the coated optical glass fibers together contacts the outer ink layer if present, or the colored outer primary coating.
When a single optical glass fiber of the assembly is to be fusion connected with another optical glass fiber or with a connector, an end part of the matrix layer can be removed to separate each of the optical glass fibers.
Desirably, the primary coatings on the coated optical glass fibers, and the ink coating if present, are removed simultaneously with the matrix material to provide bare portions on the surface of the optical glass fibers (hereinafter referred to as "ribbon stripping"). In ribbon stripping, the matrix material, primary coatings, and ink coating, are desirably removed as a cohesive unit to provide a clean, bare optical glass fiber which is substantially free of residue.
A common method for practicing ribbon stripping at a terminus of the ribbon assembly is to use a heated stripping tool. Such a tool consists of two plates provided with heating means for heating the plates to about 90 to about 120.degree. C. An end section of the ribbon assembly is pinched between the two heated plates and the heat of the tool softens the matrix material and the primary coatings on the individual optical glass fiber. The heat-softened matrix material and heat-softened primary coatings present on the individual optical glass fibers can then be removed to provide bare optical glass fiber ends, at which the fusion connections can be made. A knife cut is often used to initiate a break in the matrix material to the inner primary coating. Typically, only about 1 to 4 cm section of the matrix material and coatings on the optical glass fibers need be removed. Identification of the bare individual optical glass fibers can be made by tracing back along the bare optical fiber until the ink coating or colored outer primary coating is seen.
Ink coatings usually have a thickness of about 3 to about 10 microns and are formed from a pigment dispersed within a UV curable carrier system. The UV curable carrier system contains a UV curable oligomer or monomer that is liquid before curing to facilitate application of the ink composition to the optical glass fiber, and then a solid after being exposed to UV radiation. In this manner, the UV curable ink composition can be applied to a coated optical glass fiber in the same manner as the inner primary and outer primary coatings are applied.
Modern high speed optical glass fiber drawing towers operate at a very high speed. Thus, the ink composition must have a very fast cure speed to ensure complete cure of the ink coating on the high speed drawing tower. However, the increase in cure speed should not come at the expense of other important properties of the ink coating, such as providing suitable break out performance. In addition, ink compositions should not contain ingredients that can migrate to the surface of the optical glass fiber and cause corrosion. The ink composition should also not contain ingredients which can cause instability in the protective coatings or matrix material. Ink coatings for optical glass fibers should be color fast for decades, not cause attenuation of the signal transmission, be impervious to cabling gels and chemicals, and allow sufficient light penetration for fiber core alignment.
From the above, it is clear that optical glass fiber technology places many unique demands on radiation-curable ink compositions which more conventional technologies, such as printing inks, do not.
U.S. Pat. No. 4,629,285 discloses a method for making an ink coating on a coated optical glass fiber in which a UV curable ink is applied to a coated optical glass fiber. The ink coating is applied in a method that preserves the concentricity of the optical glass fibers. The preferred inks are pigmented semi-opaque UV curable polymeric inks. However, the ink compositions disclosed in this patent do not have a sufficiently fast enough cure speed to be used on modern high speed optical glass fiber drawing and coating towers.
Laid open Japanese Patent Application No. H1-152405 discloses a radiation-curable ink composition containing an organic polysiloxane compound. The polysiloxane compound provides the ink coating with the ability to separate more easily from the matrix material in a ribbon assembly.
Published Japanese Patent Application No. 64-22976 discloses radiation-curable ink compositions containing specific radiation-curable oligomers. The ink composition provides an ink coating having adhesion to the outer primary coating which is separable from the matrix material in a ribbon assembly.
Conventional ink coatings can have problems with concentricity. If the ink coating is not concentric, undesirable attenuation of the signal transmission may occur. Thus, there is a need for an ink composition that can be applied to a coated optical glass fiber in a concentric layer.
Ink compositions containing pigments can be very difficult to suitably cure via exposure to actinic radiation. The pigments can cause an undesirable reduction in cure speed. Thus, there is a need for a fast curing ink composition that can be used on high speed modern day optical glass fiber drawing and coating towers.
Ink coatings can have a non-uniform coloring. Therefore, there is a need for an ink composition that is capable of providing an ink coating having a uniform color.
There is also a need for an ink composition that provides an ink coating which is significantly less susceptible to causing microbending in an optical glass fiber that can lead to undesirable attenuation of the signal transmission.